The invention relates to a method for the purification of a liquid by membrane distillation, in particular for the production of desalinated water from seawater or brackish water or process water, comprising:
passing a relatively warm vaporising steam of liquid (retentate stream) over a porous membrane, vapour flowing via the pores of the membrane to the other side of said membrane, and
condensing said vapour on a relatively cool condenser surface to give a distillate stream, said condenser surface forming the non-porous separation between a feed stream to be purified and said distillate steam, which feed stream is in counter-current with the retentate son o that an appreciable proportion of the latent heat will be transferred via vapour to the feed stream, and a gas gap with a width of less than 5 mm being present between the porous membrane and the condenser surface.
Membrane distillation differs from known distillation techniques such as multi-stage flash, multiple effect distillation and vapour compression in that a non-selective, porous membrane is used. This membrane forms a separation between the warm, vaporising retentate stream and the condensed product, the distillate stream. As a consequence of a suitable choice of material (usually polypropylene, polyethylene or polytetraflorethene), the pores (diameter of between 0.00001 and 0.005 mm, usually between 0.0001 and 0.0005 mm) are not wetted by the liquid; only vapour passes through the membrane.
Membrane distillation was first described in U.S. Pat. No. 3,334,186 from 1967. The intention was to improve the efficiency of seawater desalination by the use of an air-filled porous hydrophobic membrane The method concerned here was so-called direct contact membrane distillation: the warm seawater stream and the cold distillate stream are in direct contact with the membrane.
Substantial interest in membrane distillation was generated in the mid 1980s when a new generation of hydrophobic, highly porous membranes became available. However, research showed that membrane distillation is no less expensive than competitive techniques and therefore there was no commercial application.
A distinction can be made between four types of membrane distillation:
1. Direct contact membrane distillation (DCMD), where both the warm, vaporising stream and the cold condensate stream (distillate stream) are in direct contact with the membrane.
2. Air gap membrane distillation (AGMD), where the condenser surface is separated from the membrane by an air gap.
3. Sweeping gas membrane distillation, where the distillate is removed in vapour form by an inert gas.
4. Vacuum membrane distillation, where the distillate is removed in vapour form by vacuum. This method is described only for the removal of volatile components from aqueous streams and the point at issue is not the production of a liquid distillate.
Up to now direct contact membrane distillation has attracted the most attention.
U.S. Pat. No. 4,545,862 describes a spirally wound module (with flat membranes). This was seawater stream fed in counter-current to the vaporising retentate and the seawater stream thus effectively absorbed the heat of condensation. In this patent an example is described in which a relatively high flow rate of 5.3 liters per m2 per hour is achieved with a temperture difference xcex94T between the warm retentate and the seawater of 4xc2x0 C., with an energy consumption of only 212 kiloJoule per kg distillate produced.
In addition to the use of flat membranes, the advantages of hollow fibre membranes for direct contact membrane distillation are known. As a result of the compact packing of membrane fibres, a surface area of up to 500 m2 per m3 can be obtained, which makes lower equipment costs possible. Furthermore, it has been proposed (see K. Schneider, T. J. van Gassel, Membrandestillation, Chem. Ing. Tech. 56 (1984) 514-521) to couple a direct contract membrane distillation module with a heat exchanger module in a cycle and thus to recover heat of condensation. It is found that for seawater desalination a distillate flow rate of approximately 8.5 liters per m2 per hour is obtained for a xcex94T of 14-16xc2x0 C. and a specific energy consumption of above 1,000 kJ per kg water. Since 1984 there has been little discernable progress in the state of the art in respect of DCMD.
Air gap membrane distillation was first described in 1971 in British Patent Application GB 1 225 254 A (Henderyckx). In addition to the use of an air gap, counter-current flow of feed and retentate (and thus recovery of latent heat), is already proposed In addition, AGMD was described in 1982 in German Patent Application 3 123 409 (Siemens). This application relates to the use of a gap (with a thickness of 3 mm), filled with air, or optionally a lighter gas such as hydrogen, between a flat porous membrane and a cold condensation surface. The aim was to reduce the transport of perceptible heat by conduction through the membrane. It was established experimentally that heat transport by conduction was approximately equal to that by evaporation. Moreover, it was proposed to feed incoming seawater in counter-current to the vaporising stream and thus to recover heat. The use of solar heat as a source of heat was also claimed. A theoretical case was described in which a distillate flow rate of 3.36 kg per m2 per hour was achieved with a temperature difference xcex94T of 5xc2x0 C., with a recovery of approximately 4.9% and an energy consumption of over 850 kJ per kilogram water produced.
European Patent Application 0 164 326 describes the use of an air gap with membrane distillation, the various features being constructed in the form of concentric tubes. A variant of this in which packets of flat membranes were used is described in the article Design and filed tests of a new membrane distillation desalination process (Desalination 56 (1985), pp. 345-354). It is striking that the principle of counter-current flow of seawater and retentate is abandoned, as a result of which no recovery of heat of evaporation is possible. Energy consumption figures are then also not given.
International Patent Application WO 8607585 A (1986) is based on the same model data but deduces from these that an air gap thickness of 0.2 to 1.0 mm is needed in order to achieve both a high flow rate and a low loss of perceptible heat (300 -800 kJ/kg water). No account is taken in the model of temperature falls at and in the hot and cold wall, as a result of which a far too optimistic picture is painted.
In U.S. Pat. No. 4,879,041 air gap membrane distillation is described specifically for the preparation of ultra-pure water for the semiconductor industry. Here the effect of the thickness of the air gap, when using flat membrane sheets, on mass transport and heat transport was investigated in the region between 3 and 10 mm. It was concluded from these investigations that transport is determined by diffusion at thicknesses of less than 5 mm and by free convection at thicknesses of more than 5 mm. The performances measured were moderate: maximum distillate flow rates of 3.6 kg per m2 per hour for a vapour pressure difference of approximately 20 kPa Here again no heat of condensation is recovered and it is therefore also not surprising that a few years later there was a switch back to conventional multi-stage evaporation without membranes.
The attention paid to membrane distillation decreased in the 1990s and was in the main restricted to direct contact membrane distillation and to research into sweeping gas membrane distillation and vacuum membrane distillation for the removal and extraction of volatile components from aqueous streams.
On the basis of the literature, a system without an air gap is required for membrane distillation systems with a low energy consumption. On the basis of the prior art, it is not possible to achieve an energy consumption of less than 850 kJ per kg if an air gap is used or heat recovery is employed. This is related to high temperature differences (xcex94T frequently to more than 40xc2x0) and consequently high driving forces (vapour press=re difference as a rule well above 15 kPa).
Direct contact membrane distillation systems are of simpler design and construction and in principle are less expensive than air gap membrane distillation systems and it can be seen from the prior art that the energy consumption is lower. Thus, in the light of the prior art the choice of air gap membrane distillation for inexpensive production of distilled water from seawater or brackish water is not obvious.
The aim of the invention is nevertheless to achieve a breakthrough in the performance (distillate flow rate per unit driving force) of air gap membrane distillation and thus appreciably to reduce both the costs and the energy consumption of membrane distillation systems. The aim is, in particular, to increase the performance by a factor of at least five to above 1 kg water per m2 membrane surface area per hour per kPa vapour pressure difference and to do so in combination with a loss of perceptible heat of less than 240 kJ/kg water, or less than 10% of the latent heat.
In order to achieve this objective, the method mentioned in the preamble is characterised in that a pressure which is lower than the atmospheric pressure and higher than the vapour pressure of the feed stream is maintained in the gas gap, in that the porosity xcex5 of the porous membrane is higher than 0.7, porosity being understood to be the ratio of the open volume to the total volume of the porous membrane, in that the surface area of the condenser surface is 1.2 to 6 times, preferably 2 to 3 times, the surface area of the porous membrane, in that the effective local vapour pressure difference between the retentate stream and the condensing stream is less than 10 kPa (0.1 bar), preferably less than 5 kPa (0.05 bar), and in that the perceptible heat of the distillate stream is released by heat exchange to the feed sun and for the retentate stream, with preference for the retentate stream. The perceptible heat loss of the retentate stream is less than 300 kJ/kg condensate (less than 12% of the latent heat) and the specific flow rate is higher than 0.5 kg (preferably higher than 1.0 kg) condensate/m2 membrane/hour/kPa difference in water vapour pressure.
The relationship between the porosity xcex5 of the membrane, the ratio S between condenser surface area and membrane surface area, the local vapour pressure difference D between retentate and feed, the width L of the gas gap in cm and the ratio P between the absolute pressure in the gas gap and the local water vapour pressure of the retentate is preferably as follows:             ϵ      .              S        6                    D      .      L      .              P        2               greater than       1    ⁢          (              preferably         greater than         2            )      
Known methods for AGMD and/or DCMD give a result of less than 0.5 (usually less than 0.1) for this relationship and also do not yield the desired performance.
When employing the method according to the invention, use can advantageously be made of a number of module segments connected to one another and each formed by a number of porous retentate channels, connected in parallel, which are separated by a gas gap and a non-porous membrane from feed steam channels which are positioned at an angle with respect to the retentate channels.
Said angle between the retentate and feed stream channels is between 10 and 170xc2x0.
The retentate channels are usually delimited by porous hydrophobic membranes (porosity greater than 70% and preferably greater than 80% and pore size larger than 0.1 xcexcm, preferably between 0.3 and 1.0 xcexcm). The membranes concerned can be commercially available membranes made of materials such as PTFE, PVDF, PP and PE and the like. So-called asymmetric microfiltration membranes made of materials such as polyethersulphone, polysulphone, polyacrylonitrile, polyamides, etc. can also be used. In this context it is preferable to make the surface of these membranes completely or partially additionally hydrophobic, for example by means of a coating or other surface modification. In the simplest embodiment the retentate channels consist of hollow fibres or capillary membranes placed in parallel. The retentate flows through the lumen of these fibres When asymmetric membranes are used the active layer of the membrane, with the narrowest pores, is on the retentate side.
In addition to hollow fibres, however, the retentate channels can also be formed by flat plate membranes or membrane sheets, optionally in a spirally wound configuration. In principle it is also possible for the retentate channels to be formed from hydrophilic material, such as threads or fabric, or for non-porous (but highly vapour-permeable) membranes to be used.
The condenser channels, through which the feed stream runs, are likewise preferably formed by hollow fibres/capillaries made of hydrophobic material, placed in parallel. These channels are now non-porous, that is to say are not vapour-permeable or are barely vapour-permeable. The discharge of the condensate distillate can take place via hydrophilic material (such as a fabric) that has been applied to or around these fibres. It is also possible to produce the condenser channels from hydrophilic material around which a film of distillate forms which, for example, can be discharged by gravity.
According to the invention the condenser surface area must be larger than the surface area of the retentate channels (1.2-6 times, preferably 2-3 times larger). This can be achieved by positioning the capillaries close to one another and/or by using multiple rows (as a rule two or three).
The width of the gap between the retentate channels and condenser channels, the so-called gas gap, can be defined by using a suitable spacer, preferably made of hydrophobic plastic. In order to restrict loss of perceptible heat from retentate to the feed by conduction through this material, the material must be highly porous (porosity preferably greater than 90%). The thickness of the material determines the width of the gas gap: less than 5 mm, preferably between 0.5 and 2 mm.
Preferably the gas gap is in particular filled with water vapour by producing a vacuum in the gas gap such that this vacuum is approximately equal to the vapour pressure of the retentate stream in situ in the module segment concerned. This gives the best performance with regard to mass transfer (water vapour transport from retentate to condenser) and limitation of the loss of perceptible heat. This vacuum can be produced by using a vacuum pump which operates on the bottom, and thus the coolest, module segment where the lowest assure will prevail, and by fitting resistances to flow, which may or may not be controllable, between all or some of the module segments. It is also possible for the condensate, possibly containing gases, to be discharged by means of a suction pump per module segment The gas gap can, however, also be at or around atmospheric pressure and filled with an inert gas, such as air or, preferably (in connection with the mass transfer and heat transfer), hydrogen or helium. Carbon dioxide, nitrogen and similar gases can also be used.
The process according to the invention can be implemented in a number of embodiment. Usually use will be made of cross-flow module segments equipped with hollow fibres for both the feed stream and the retentate stream. However, flat membrane sheets or plates, optionally in a spirally wound configuration, or plate-like structures consisting of multiple capillary-like channels to limit the flow channels can also be used for the retentate stream.
In a first embodiment of the method, the feed stream to be heated flows in virtually ideal counter-current to the vaporising retentate stream and the condensate distillate steam flows in co-current with the retentate stream and/or in counter-current to the feed stream, by which means the perceptible heat is also recovered.
In a variant of this, the retentate stream is alternately in heat exchange with a residual heat stream and the feed stream. This variant has the advantage that fill use is made of the residual heat and that the driving force at the inlet side of the feed stream is maintained by a rising temperature difference. The modules suitable for this can be of extra compact and inexpensive construction.
With this alternative embodiment it is possible for the retentate stream also to be brought into heat exchange with a stream of cooling water in accordance with the principle of the first embodiment of the method, an additional distillate stream being formed.
In a second variant the retentate stream is used, after it has been cooled by discharge of heat of evaporation to the feed stream, as a condenser by feeding this retentate stream through vapour-tight channels in cross-flow with the same retentate stream for cooling. The distillate stream is thus formed both by cooling by means of the feed strep and by cooling by means of the retentate stream.
The invention also relates to a module suitable for use with the method described above. Such a module is made up of a number of segments joined to one another, each segment consisting of layers of essentially parallel non-porous fibre membranes for the feed stream and layers of essentially parallel porous fibre membranes for the retentate stream, which porous fibre membranes are at an angle of between 10xc2x0 and 170xc2x0 to non-porous fibre membranes, a layer of porous fibre membranes being arranged between each two successive layers of non-porous fibre membranes, which layer of porous fibre membranes is held some distance away from the layers of non-porous fibre membranes by means of spacers, which distance is less than 5 mm. Each segment has a distribution chamber for feed liquid to be supplied, a distribution chamber located opposite the latter for feed liquid to be discharged, a distribution chamber for retentate to be supplied and a distribution chamber opposite the latter for retentate to be discharged.